Magnetic Nanowires from DNA
William Ballik, Robbie Edwards, Alex Klotz, Gio Mitchell
Introduction
One of the foremost areas of nano research
is into nanowires, nanorods, and nanotubes,
the classic example being carbon nanotubes:
these nearly one-dimensional structures have
one long dimension, perhaps a few microns
long and two short dimensions, on the order
of nanometers. One relatively unexplored
area is magnetic nanotubes, which could be
used for applications similar to other
magnetic media, such as memory on a very
small scale or magnetic field sensors. Of
particular interest are magnetic nanotubes
where the diameter is less than or equal to
the width of the so-called “domain wall” of
the materials, a magnetic property of certain
metals, which occur at the 10-50 nm
thickness scale.
There exist fabrication techniques for
metallic nanoparticles, involving, for
example, electroplating, where an electric
current is applied through a solution to cause
the deposition of metal particles.
Nevertheless, it is extremely difficult to
ensure that the width of the material formed
remains small, and to control it. This is
where biological material comes in: Nature
has, of course, had many more years of
practice at building small-scale phenomena
than Man has. Using existing biomolecules
as a type of “backbone” for nanotubes, and
in particular magnetic nanotubes, has been
done using microtubules and peptides. Still,
perhaps the most promising is double-helix
DNA, which has several properties that are
ideal for nanotubes: a very narrow width (on
the order of 2 nm), a very straight line
(without any branching into different
directions), and an efficient self-assembly
mechanism (using base-pairs). It is because
of this that Qun Gu, Chuanding Cheng, and
Donald T. Haynie of Louisiana Tech
University have used DNA to form 10-20
nm high magnetic cobalt-based nanotubes.
Figure 1: Atomic Force Microscopy image of DNA on mica
substrate. The width of DNA is 1-2 nm; the white line is 1
um.
From DNA to Cobalt Nanowire
Bacteriophage DNA was used in all
experiments; an image of the DNA before
nanotube creation is recorded in Figure 1.
Two different materials were used as the
substrate, or base plate, for the nanotube
growth: mica and silated glass (glass with
some silicon added). These needed to be
prepared in different ways, but both
followed a general three-step process. The
DNA was first “activated” using palladium
ions, which attach to some of the bases in
the helix. Then a biochemical called DMAB
was used to reduce (de-ionize) the palladium
ions on the DNA. And finally, cobalt ions
are added in the solution and attach to the
DNA with the palladium acting as a catalyst.
This process is called electroless as it does
not require an external electric potential to
operate. The use of the palladium allows for
high uniformity of cobalt and a great deal of
control over where the cobalt goes: the
cobalt will only attach to the parts of DNA
coated with palladium.
The palladium
activation took place over an incubation
period of about 24 h at body temperature,
and the actual cobalt plating took place over
a period of about 10-15 minutes at 50
degrees Celsius.
Additionally, control
systems were produced of “unactivated”
DNA (without the palladium ions added)
reacting with palladium and cobalt in
solution, as a basis of comparison.
Experimental Results
The electroless plating led to 10-20 nm high
clusters of cobalt on the DNA surface, more
or less uniformly distributed along the DNA.
An image of the cobalt atoms taken with
atomic force microscopy is shown in Figure
2. A second treatment of the DNA in a
cobalt bath allowed the nanowires to grow
Figure 3. Atomic Force Microscopy characterization of
cobalt wires, in the unactivated (A) and activated (B) cases.
The vertical scales are 5 and 11nm respectively.
even further to 25 nm. Unsurprisingly, the
palladium ion-activated DNA fared much
better in attaching cobalt clusters (compare
the images of figure 3-A and 3-B);
the metal coverage was approximately 7080% without the activation, which is not
enough to form a continuous nanowire.
In order to “straighten out” the cobalt-plated
DNA, a so-called “molecular combing”
process was used. This process involves one
end of the DNA is anchored onto the glass
substrate, and then evaporating away the
water in the solution so that the other end of
the DNA stretches along with the water
meniscus. An image of the DNA cobalt
nanotubes combing is shown in Figure 4.
Figure 4: AFM image of cobalt wires after molecular
combint was used to orient them.
Conclusions
Figure 2: Atomic Force Microscopy characterization of
Cobalt wires. The screen area is 1 um x 1 um, and the
height is approximately 14 nm.
DNA was treated with palladium and
coating in cobalt to produce magnetic
metallic nanowires. The method was
electroless and the wires could be oriented
by molecular combing. Metal plating of
DNA is a useful technique for creating
nanotubes. Before these can be used as
memory devices or other applications,
directionality has to be controlled, but this
represents an important step towards
magnetic nanowires.
References
Gu, Qun; Chuanding Cheng; Donald T.
Haynie. “Cobalt metallization of DNA:
Towards
magnetic
nanowires.”
Nanotechnology 16 (2005) 1358-1363.